1. Field of the Invention
This invention relates to a laminated chip common mode choke coil and, more particularly, to the same which is used for a high frequency circuit.
FIG. 6 is an exploded perspective view showing processes of manufacturing a conventional laminated chip common mode choke coil. In this conventional example, the laminated chip common mode choke coil having a winding of 2.5 turns is formed by laminating magnetic substance sheets 31a-31n provided with a conductor pattern and by pressing and thereafter baking them into an integral entity. Ferrite sheets 31b, 31d-31k and 31m are provided with a conductor pattern and a through-holes. The number of turns of this laminated chip common mode choke coil can be increased by repeatedly laminating ferrite sheets 31f-31i for example.
On ferrite sheets 31a and 31n, no conductor pattern is formed. On the surface of the ferrite sheet 31b, a conductor pattern 32 is formed which becomes a lead-out electrode at one end of one coil. One end of the conductor pattern 32 is drawn out to the outer edge of the ferrite sheet 31b, and the other end extends to a through-hole 32a. Following the ferrite sheet 31b, a ferrite sheet 31c provided with a through-hole 33a is laminated.
A conductor pattern 34 which becomes one end of the other coil is formed, roughly in a U shape, on the ferrite sheet 31d. One end of the conductor pattern 34 is led out to the outer edge of the ferrite sheet 31d to be formed into a conductor pattern 34a that becomes a lead-out electrode, and the other end extends to a through-hole 34b. Furthermore, on the ferrite sheet 31d, a through-hole 34c which connects to the through-hole 33a is formed in a position not contacting with the conductor pattern 34.
On the ferrite sheets 31e, 31f, 31g, 31h, 31i and 31k, conductor patterns 35, 36, 37, 38, 39 and 41 are formed respectively, and through-holes 35a, 35b, 36a, 36b, 37a, 37b, 38a, 38b, 39a, 39b and 41a are also formed in the above ferrite sheets.
On a ferrite sheet 31j, a conductor pattern 40 which becomes the other end of the other coil is formed roughly in an L shape. On end of the conductor pattern 40 is led out to the outer edge of the ferrite sheet 31j to be formed into a conductor pattern 40a that becomes a lead-out electrode. On the ferrite sheet 31j, a through-hole 40b which connects to the through-hole 39a is formed in a position not contacting with the conductor pattern 40.
A conductor pattern 43 is formed which extends to the outer edge of the ferrite sheet 31m and becomes the lead-out electrode of the other end of one coil.
In the choke coil, one coil comprises the conductor patterns 35, 37, 39 and 41, and both ends of it are connected to the conductor patterns 32 and 34 respectively. And the other coil comprises the conductor patterns 34, 36, 38 and 40, and both ends Of it are connected to the conductor patterns 34a and 40a respectively.
As shown in FIG. 7, in the choke coil, a rectangular coil of 1.0 turn is formed with the ferrite sheets 31f, 31g, 31h and 31i. As shown in FIGS. 8 or 9, a hexagonal coil 45 or a circular coil 46 each having a different pattern from the rectangle may be formed.
In a common mode choke coil, normal mode impedance may lead to attenuation of a signal, and thus it is desirable to suppress the impedance to a low level. However, among conventional chip type common mode coils such as the above-mentioned, a good common mode impedance characteristic and a good resistance characteristic could be obtained, while a normal mode impedance characteristic at a high frequency (30-150 MHz) is not good. As shown in FIG. 5, a choke coil having the coil pattern of FIG. 7 causes an impedance of 110.OMEGA. at a frequency in the vicinity of 50 MHz.
A choke coil having another coil pattern such as shown in FIGS. 8 or 9 similarly has a problem of increase in normal mode impedance although it satisfies other characteristics. It was found that a portion not contributing to coupling between the two coils exists in the lead-out electrode as the cause of increase in normal mode impedance.